In the 20 years since cleanup of contaminated groundwater has been
a high priority in the United States, recognition of both the scope
of the problem and the technical difficulties involved has grown steadily.
Estimates of the number of hazardous waste sites where groundwater
may be contaminated vary between 300,000 and 400,000 nationwide [NRC,
1994]. Legislation passed in the 1980s by Congress and the states
generally required that groundwater in contaminated aquifers be restored
to background or drinking water standards. Unfortunately, attempts
to meet these goals using conventional methods, such as pump and treat
systems, frequently have been unsuccessful [NRC, 1994].

Moreover, constructing and operating engineered groundwater cleanup
systems is very expensive, with total spending on environmental remediation
in the United States estimated at $9 billion in 1996 alone [NRC, 1997].
Over the last 5 years, the high costs of engineered cleanup systems
and their disappointing performance has spurred both a search for
alternative remediation strategies and rethinking of remediation goals
and time frames. In this climate, interest in the capabilities of
a remediation strategy known as monitored natural attenuation has
skyrocketed.

Fig. 1. Conceptual
illustration of the important natural attenuation processes that affect
the fate of petroleum hydrocarbons in aquifers

Use of monitored natural attenuation as a remediation strategy involves
filing a formal regulatory application to allow natural biological,
chemical, and physical processes to treat groundwater contaminants,
and conducting ongoing monitoring to verify that these processes are
effective. In many cases, the feasibility of a natural attenuation
strategy depends on whether the regulatory goal is to clean up the
contaminant plume to drinking water standards or whether a less stringent,
risk-based goal applies, such as preventing the plume from expanding.
Since 1995, the use of natural attenuation as a remedial solution
for benzene, toluene, ethylbenzene, and xylene (BTEX) has increased
dramatically [NRC, 2000]. More recently, natural attenuation has been
proposed for chlorinated solvents, nitroaromatics, heavy metals, radionuclides,
and other contaminants for which the scientific understanding and
field experience are much less robust.

Groundwater remediation by natural attenuation is controversial.
Environmental advocates charge that natural attenuation is little
more than an excuse for industry to avoid the high costs of building
cleanup systems. To address these issues, in 1997 the National Research
Council (NRC) appointed the Committee on Intrinsic Remediation to
provide guidance on when and how to determine whether natural attenuation
is an appropriate remedy for a site. Community leaders interviewed
by the NRC committee expressed concern that natural attenuation allows
responsible parties to save on cleanup costs while shifting the risks
to the community. Furthermore, they expressed mistrust of the multi-process
definition of natural attenuation and felt that reductions in contaminant
concentration by dilution or transfer to another medium might continue
to pose risks.

Community leaders indicated a willingness to accept natural attenuation
when responsible parties and regulators can provide solid evidence
that natural attenuation processes are transforming the contaminants
to harmless products. The NRC committee's recently released report,
Natural Attenuation for Groundwater Remediation [NRC, 2000], concluded
that the key to demonstrating the effectiveness of natural attenuation
at a site is establishing the cause-and-effect relationship between
loss of contaminant and the mechanism responsible for the loss. This
conclusion has important implications for future research and training
in the areas of hydrogeology and subsurface biogeochemistry.

The NRC report emphasizes that the success of natural attenuation
depends on both the nature of the contaminant and the specific subsurface
conditions at a site. In documenting natural attenuation at a contaminated
site, a hydrogeologist must develop a substantial degree of understanding
of the subsurface processes. Thus, the major expense of the remediation
may shift from the design and operation of an active system to detailed
investigation and monitoring of the site. The goal of the site investigation
is to understand the natural groundwater flow and biogeochemical reactions
responsible for controlling the contamination.

The NRC report recommends a varying level of investigation depending
on the hydrogeologic complexity of the contaminated aquifer and the
nature of the contaminants. More complex environments require a greater
degree of characterization and quantitative mass balance. An increased
level of characterization also is required when natural attenuation
processes are effective only under specialized geochemical conditions.
The processes controlling the subsurface fate of many contaminants
are only partially understood and are still topics of active research.
Thus, the committee's report emphasizes that broad training in hydrogeology
and biogeochemical fundamentals is essential for future employment
in the environmental geosciences. This training provides the skills
to understand unique contaminants and situations outside the scope
of current knowledge.

Much of the theoretical and practical basis for natural attenuation
as a remediation strategy has been based on large interdisciplinary
studies at field sites. The results of these studies are providing
essential insights into the effects of site conditions on the fate
of subsurface contaminants. Subsurface geochemical settings that favor
natural attenuation for some contaminants are unfavorable for other
contaminants. Moreover, geochemical and hydrogeological conditions
can change with time and location in the subsurface.

Because of the importance of understanding the effects of site conditions
on natural attenuation processes, the NRC committee report states
that training of environmental geologists should include descriptions
of comprehensive natural attenuation case studies. In accord with
this recommendation, the main points of the NRC report are illustrated
with results from field studies of natural attenuation processes for
a variety of compound classes and sites. Two of the case studies presented
in the report are based on results from the U.S. Geological Survey's
Toxic Substances Hydrology Program (USGS Toxics Program) research
sites. The goal of research conducted by the USGS Toxics Program has
been to arrive at general scientific principles to guide investigations,
monitoring strategies, and regulatory decisions on the effects of
natural processes on environmental contaminants. The remainder of
this article reviews the results of two USGS Toxics Program case studies
in the context of the NRC committee's recommendations. This is followed
by a discussion of the emerging research issues for natural attenuation
processes.

Table1: Sites
where USGS researchers and university collaborators are conducting intensive
field investigations of representative cases of subsurface contamination
and solute transport as part of the USGS Toxics Substances Hydrology
Program.

Detailed case studies illustrate how site investigations can reduce
uncertainty and document the natural attenuation processes responsible
for the reduction of contaminants. The contaminants at the USGS Toxics
Program subsurface research sites cover a range of contaminant classes
(Table 1). The fate of each contaminant class in the subsurface is
determined by its chemical properties and the environmental conditions
at a site. A major distinction exists between organic chemicals and
metals. While naturally occurring biodegradation can completely convert
some organic contaminants to harmless products, metals can only be
transformed to forms that are less mobile or toxic.

To illustrate this point, we present two case studies: one on the
fate of organic contaminants and another on the fate of metals.
Frequently, the effectiveness of natural attenuation can be documented
using "footprints" of the underlying mechanisms. Footprints
are changes in concentrations of reactants and products that are
diagnostic of specific biogeochemical reactions acting to transform
or immobilize the contaminant. Coupling the observed loss of contaminant
to the measurement of several footprints demonstrates which natural
attenuation processes may be responsible for contaminant loss. At
sites where the contaminant source will be present for many years
into the future, it may be necessary to determine the quantities
of important natural attenuation reactants in the aquifer. A mass
balance is then performed to ensure natural attenuation is sustainable
for the lifetime of the source.

The Fate of Crude Oil

A buried oil pipeline located in a glacial outwash plain near Bemidji,
Minnesota, ruptured in 1979 and an estimated 3,200 barrels of spilled
oil infiltrated the subsurface. The oil forms a long-term, continuous
source of hydrocarbon contaminants that dissolve in and are transported
with the groundwater (Figure 2a). Microbial degradation of the petroleum
hydrocarbons in the plume has resulted in the growth of aquifer microbial
populations dominated by iron-reducers, fermenters, methanogens, and
aerobes (Figure 2b). The biodegradation reactions cause a number of
geochemical changes or diagnostic footprints near the aqueous plume.
These include decreases in concentrations of oxygen and hydrocarbons
and increases in concentrations of dissolved iron, manganese, and
methane (Figure 2c). Characterizing the various biodegradation processes
occurring in the aquifer is important because each process results
in different degradation rates for the individual hydrocarbon compounds.

Simulations of the evolution of redox zones and microbial populations
in the plume provide important insights, including estimates of losses
due to each biodegradation process and the long-term sustainability
of the hydrocarbon degradation. A vertical cross-section parallel
to the direction of groundwater flow was simulated from the time of
the spill in 1979 until September 1992 using the code BIOMOC [Essaid
and Bekins, 1997]. In the model, aerobic, Mn and Fe reducing, and
methanogenic aquifer microbes degrade the dissolved hydrocarbons.
Aerobic degradation takes place first, and oxygen inhibits anaerobic
processes. As oxygen is consumed and an anoxic zone develops, the
Mn/Fe reducers and methanogens begin to grow, consuming solid-phase
Mn(IV) and Fe(III), and releasing dissolved Mn(II), Fe(II), and methane.

The model calibration involved balancing the observed spatial and
temporal variations of hydrocarbons against the other degradation
reactants and the observed microbial populations. Steady-state flow
was assumed, and literature values, theoretical estimates, and field
biomass measurements were used to obtain reasonable estimates of the
transport and biodegradation parameters. Simulated concentrations
and data in Figure 2c illustrate how the evolution of redox zones
results in changes in water chemistry over time. The simulation predicts
that 60% of the hydrocarbon degradation occurs anaerobically (Mn reduction:
5%, Fe reduction: 19%, methanogenesis: 36%) and 40% occurs aerobically.
The field data, modeling, and microbial population results illustrated
in Figure 2 suggest that the natural attenuation capacity of the aquifer
near the oil is being slowly consumed. This has been confirmed by
monitoring of the site over the 20 years since the spill [Cozzarelli
et al., 2001].

The Fate of Metals from Mine Waste

Acidic drainage from former mine sites is the most frequent cause
of metal-contaminated groundwater, with an estimated 20,000-50,000
affected sites nation-wide. In the Pinal Creek Basin of central Arizona,
large-scale copper mining since the late 1880s and related activities
have resulted in contamination of an alluvial aquifer by acid mine
drainage. The plume of acidic water extends 25 km down gradient from
the location of mining operations at the head of the basin (Figure
3) and contains excessive concentrations of iron, manganese, copper,
zinc, aluminum, cobalt, and nickel. The source of the plume probably
had a pH of 2-3 and iron and sulfate concentrations exceeding 2000
and 19,000 mg/L, respectively.

Reactions between the acid mine drainage plume and the aquifer materials
have resulted in several footprints of natural attenuation processes.
These include carbonate and manganese oxide dissolution, pH increase,
increase in calcium, and decreases in concentrations of iron, copper,
and aluminum. As the plume migrates through the aquifer, oxidation
of reduced iron by sediment manganese oxides results in iron precipitation
and release of more dissolved manganese. In addition, reaction of
the acidic water with carbonate minerals and sorption of hydrogen
ions on the precipitated iron increases the pH to 5-6 [Stollenwerk,
1994]. The sorption reactions of aqueous copper, cobalt, nickel, and
zinc depend strongly on pH. Thus, at the location where the pH increases
in the plume, sorption by iron oxides removes substantial quantities
of copper from the groundwater and the migration rates of cobalt,
nickel, and zinc are retarded. The pH increase also causes aluminum
to precipitate onto the aquifer solids. Because chloride was present
in the plume source water, it is possible to assess the role of dilution
in decreasing the contaminant concentrations.

Fig. 2.At the crude
oil study site near Bemidji, Minnesota, aerobic and anaerobic biodegradation
are the most important natural attenuation processes affecting the hydrocarbon
plume. a) Contour plot of 1995 total BTEX concentration in the Bemidji
plume for a vertical cross-section along the plume axis [modified from
Cozzarelli et al., in press, 2001]. b) Cross-section showing the distribution
of physiologic types of microorganisms inferred from most probable number
data [modified from Bekins et al., 1999]. The areas contaminated by
separate-phase oil are predominantly methanogenic in both the saturated
and unsaturated zones. c) Simulated and observed concentrations versus
time for a water table well located 36 m down gradient from the center
of the oil body. The loss of oxygen and production of reduced electron
acceptors (Fe(II), Mn(II), and methane) illustrate the temporal evolution
of redox conditions in the aquifer [modified from Essaid et al., 1995].

Fig. 3.Cross-section
of Pinal Creek Basin showing the pH of the groundwater contaminant plume
migrating from the former location of mining and related activities
at the far left [modified from Stollenwerk, 1994]. Groundwater flowing
beneath the contaminant source area has a pH less than 4 (as shown on
the dashed line marked 4), but farther down gradient the pH increases
to 5-6 as carbonate minerals neutralize the acidity. The low-pH region
corresponds to the region with high concentrations of dissolved metals.
Attenuation of the dissolved metals by sorption and precipitation occurs
as the pH increases. Oxidation or sorption of metals in the streambed
further attenuates metals that reach the perennial stream

The acidity of the plume has resulted in depletion of the carbonate
in the aquifer and related slow migration of the front edge of the
low pH plume at a rate of about one-seventh of the advective groundwater
flow. The metal sorption reactions are reversible, causing remobilization
of the metal contaminants as the pH front migrates down gradient.
Although the acidic front of the plume has not yet reached Pinal Creek,
the leading edge of the neutralized plume, containing elevated concentrations
of manganese, cobalt, nickel, and zinc, discharges into the perennial
reach of the creek. Within the creek, the pH and dissolved oxygen
of the contaminated water increase due to gas exchange with the atmosphere.
Precipitation of manganese oxides on the stream sediments is enhanced
by the presence of manganese-oxidizing bacteria, which immobilizes
about 20% of the dissolved manganese flowing out ofthe drainage basin
[Harvey and Fuller, 1998].

Sorption onto the manganese oxides also reduces the dissolved mass
of nickel, zinc, and cobalt in the stream by 12-68% depending on the
type of metal [Fuller and Harvey, 2000]. Dilution over a 7-km perennial
reach of the stream results in an additional 20% decrease in concentrations
of the dissolved metals. Although natural attenuation in the aquifer
and stream decreased metal transport, the size of the plume and the
longevity of the source of contamination at Pinal Creek Basin have
overwhelmed the intrinsic remediation capacity of the aquifer. However,
similar processes govern the fate of metals at other acid mine sites,
substantially reducing the mobility of metals in aquifers and streams
that receive metal-contaminated, ground-water discharge.

Emerging Research Issues

With the dramatic increase in the use of natural attenuation, there
are many issues that require further study to ensure that this strategy
is protective of public health and the environment. These fall into
three broad categories: poorly understood chemical classes, uncertainty
posed by subsurface heterogeneity, and long-term sustainability. An
important conclusion of the NRC report is that natural attenuation
has been demonstrated to be effective most of the time for only a
few compounds. There are many compounds for which the natural attenuation
potential has not yet been established. For example, the gasoline
oxygenate MTBE is highly mobile in the environment and the rates of
physical, chemical, and biological processes controlling its environmental
fate are the subject of active research by the USGS Toxics Program
and others. Another important area of research is the natural attenuation
potential for mixtures of contaminants such as those found in the
Norman landfill plume (Table 1).

Because the migration paths and degradation potential of groundwater
contaminants are strongly affected by subsurface physical and chemical
heterogeneity, the NRC report recommends that increased effort is
needed to document the effectiveness of natural attenuation where
the subsurface is highly heterogeneous. Thus, strategies for characterizing
the effects of subsurface heterogeneity on natural attenuation and
reducing uncertainty in contaminant risk assessments are also fertile
areas of research. The application of quantitative modeling at many
USGS Toxics Program sites is providing examples of how uncertainty
can be assessed and used to guide new data needs.

Because documenting the fate of solutes in fractured rock aquifers
is particularly difficult, developing new methods for characterizing
flow and transport in fractured rock is the goal of research at the
Mirror Lake study site (Table 1).

Finally, the NRC report stresses that many practical issues regarding
the performance of natural attenuationover long time frames are
still unclear. These include the effects of active remediation efforts
on the natural attenuation processes; the design of long-term monitoring
networks to verify that natural attenuation is working as expected;
and the natural attenuation capacity of the aquifer over the lifetime
of the source. The effect of source removal on natural attenuation
processes is the focus of USGS Toxics Program research efforts at
the Bemidji crude oil and the Cape Cod treated sewage plume sites.

As natural attenuation sites nationwide are monitored over the
coming decades, the results of detailed process studies from the
USGS Toxics Program and other field-based research efforts will
provide a framework for assessing the continued sustainability of
natural attenuation. Further information on the USGS Toxics Program,
site summaries, and reference lists may be found at http://toxics.usgs.gov/.
The NRC report is available at http://books.nap.edu/.